1. Technical Field
This disclosure relates to spectroscopy, and more particularly, to systems and methods for detecting or determining characteristics of materials under paints or coatings by employing early light spectroscopy.
2. Description of the Related Art
Paint or other coatings are typically applied to surfaces to protect the surfaces against corrosion or other damage. In some instances corrosion, cracking or other damage begins under the paint or coating and is undetectable by visual inspection. Inspections may be carried out by one or more of the following non-destructive imaging (NDI) techniques.
Ultrasonic (pulse echo or through transmission) methods can monitor larger defects of, for example, aircraft structures for a whole-field but are not preferred for detection of early or surface deterioration. The techniques are particularly conducive to rapid imaging of a surface and include magneto-optic eddy current imaging, active thermograph, optically aided visual inspection, and spectral imaging.
Magneto-optic imaging (MOI) can image corrosion and cracks over a small area the size of the magneto-optic crystal plate used in a hand-held scanner. However, MOI""s sensitivity to top-surface corrosion depends on the degree to which the eddy currents are altered and gives rise to anomalies in the induced magnetic field at the surface. It has not been shown conclusively that the MOI technique can detect incipient corrosion that has not yet produced a significant increase in macroscopic surface roughness.
Active Thermography (AT) is an increasingly important technique for detecting subsurface flaws such as delamination, debonding, and second-surface corrosion. AT has a lower sensitivity to incipient corrosion under paint; however, because (1) the initial stages of corrosion do not significantly increase the thermal impedance of the surface compared to a layer of paint alone, and (2) the detailed resolution of incipient corrosion effects at the top surface requires a very high speed infrared camera to resolve surface transients which may appear only in the few milliseconds after the initial flash lamp illumination. Such equipment is prohibitively expensive for use on a wide scale. However, for significant surface corrosion, AT and MOI have the potential to image significant surface corrosion damage and distinguish it from subsurface effects by employing commercially available instrumentation.
Visual Inspection (VI) is used to determine the extent of corrosion damage on a skin of a surface and around fasteners, for example, after the paint or coating has been stripped. As a nondestructive technique for painted aircraft, visual techniques are not amenable to detection of chemical changes or micro-roughness at the paint/metal interface, unless significant corrosion products penetrate through the thickness of the paint. Specialized primers and paints, which incorporate taggants, are still in the research stage and not in general use.
Spectral Imaging (SI) techniques use a compact multi-spectral imaging sensor. This method is based on the partial transparency of many aircraft paints to specific bands of infrared radiation. Using this method, it is possible to detect changes in the chemistry of the metal surface or the primer by analyzing the amplitude of reflected and emitted radiation at specific wavelengths. The layered depth information cannot be deduced from a simple SI approach, however.
Therefore, a need exists for a system and method for inspecting surfaces through a coating, such as paint. A further need exists for such a system and method, which improves upon existing techniques and does not suffer from the disadvantages as described above.
The present invention includes near infrared (NIR) optical imaging systems and methods to non-destructively image (NDI) deteriorations or defects in painted metals and artwork beneath painted surfaces are disclosed. Back-scattered light is used to determine its suitability to monitor corrosion and cracking in metal beneath paints, up to a thickness of about 500 xcexcm. NIR light, in the paint transmission zoning spanning from 800 nm to 10,000 nm can be used to assess the quality of metallic structures below the paint level for incipient and advanced stages of corrosion and cracking. NIR light scattered from paint, corrosion, air voids, and metal can be spatially imaged in micrometers sliced sub-surface layers. Art work below a painted overcoat can be imaged and detected. Spectral, temporal, spatial, nonlinear optical, and polarization gates are employed to distinguish phantoms in turbid media, such as painted corroded metal and cracked specimens: such as painted surfaces from airplanes, submarines, ships, automobiles, and bridges.
In a layered paint-metal medium, the paint acts as a highly scattered pigmented medium where high scattering occurs. Transmission windows exist in NIR region. The metal acting as flat mirror reflects the scattered light. When corrosion or cracking is present, the smooth metal becomes microscopic irregular, which adds scattering and time delay of the probing light.
A system for non-destructively imaging surfaces through a coating, in accordance with the present invention, includes a near-infrared (NIR) light source for illuminating a coated surface. A detector is positioned in an operative relationship with the NIR light source to receive light backscattered from the coated surface and from the coating. A gating device is positioned in an operative relationship with the detector to selectively permit light to pass to the detector to measure optical characteristics of the backscattered light such that determinations of a state of a surface below the coating is determined based on the optical characteristics of the backscattered light.
A method, in accordance with the present invention, for non-destructively imaging surfaces covered by a coating includes the steps of irradiating a coated surface with near-infrared light to provide backscattered light from the coating and the coated surface, gating the backscattered light to selectively permit early light to pass to a detector, and measuring optical characteristics of the early light received by the detector to image the coated surface below the coating to determine if damage exists on the coated surface.